Examples of an alkaline-earth metal silicate phosphor well known as a phosphor material for white LEDs include a yellow-emitting phosphor having a compound phase represented by a compositional formula, Sr3SiO5:Eu, (Sr, Ba)3SiO5:Eu, or (Sr, Ba, Ca)3SiO5:Eu, a green-emitting phosphor having a compound phase represented by a compositional formula, (Ba, Sr)2SiO4:Eu or (Ba, Sr, Ca)2SiO4:Eu, and an orange- to red-emitting phosphor having a compound phase represented by a compositional formula, (Sr, Ca)2SiO4:Eu or (Sr, Ca, Mg)2SiO4:Eu.
These strontium (Sr)-containing alkaline-earth metal silicate phosphors are used for high-brightness or high-color-rendering white LED devices, and absorb part of exciting light from a blue LED and emit yellow light, green light, and orange to red light, respectively. Further, neutral white light or warm white light can be more efficiently obtained by mixing blue exciting light and yellow light, blue exciting light, green light, and orange-to-red light, or blue exciting light, yellow light, and orange-to-red light.
Currently, LED devices are used for various purposes such as lighting, in-car lights, and backlights for liquid crystal display televisions. LED devices used for such purposes are mainly required to have high brightness and high chromaticity, and particularly, there has recently been a demand for LED devices having higher brightness. The properties of such LED devices depend on phosphors. In order to increase the brightness of LED devices, phosphors are required to have improved light-emitting characteristics. The same applies to the above-described silicate phosphors that emit yellow, green, or orange-to-red light. However, it is not easy to improve the light-emitting characteristics of phosphors themselves, and therefore various studies have been made to improve the characteristics of phosphors.
Meanwhile, it is known that an alkaline-earth metal silicate phosphor that emits yellow, green, or orange-to-red light is degraded by formation of a hydrate or a carbonate on the surface of particles of the phosphor because an alkaline-earth metal component, such as strontium, that is a constituent element of the phosphor is eluted from the inside of the particles with water vapor in air or water. Due to such properties, the silicate phosphor that emits yellow, green, or orange-to-red light has a problem that the silicate phosphor is degraded due to long-term use in the atmosphere or temperature rise caused by exciting light so that a brightness reduction and a color change occur.
It is an important issue to be resolved for current silicate phosphors for LEDs to overcome both the above-described problems, that is, to improve light-emitting characteristics and to prevent degradation by moisture. As one of measures against the issue, a method for modifying or coating the surface of silicate phosphor particles has been proposed. For example, Patent Literature 1 discloses, as a method for chemically modifying the surface composition of phosphor particles, a method in which cations on the surface of phosphor particles are replaced with selected cations by an ion-exchange reaction. Patent Literature 1 states that a phosphor obtained by this method can have improved adhesion and an improved lumen maintenance factor and can prevent the deposition of impurities.
More specifically, a layer is formed on the surface of phosphor particles by replacing cations of a phosphor material with other cations selected from aluminum, barium, calcium, lanthanum, magnesium, strontium, yttrium, zinc, titanium, tantalum, boron, and silicon. This method is simple but has not led to a significant improvement in water resistance or moisture resistance. Further, the most serious problem of this method is that when an obtained silicate phosphor poor in water resistance is introduced into a solution, an alkaline-earth metal as a constituent element of the phosphor is eluted from the inside of particles of the phosphor so that the phosphor is degraded and its light-emitting characteristics are deteriorated.
Further, Patent Literature 2 discloses a method for producing coated silicate phosphor particles, in which a silicate phosphor for fluorescent lamps is introduced into a solution containing a barium salt or a strontium salt, the solution is stirred to bind the salt to the surface of the phosphor, and then the silicate phosphor is heat-treated. More specifically, a considerable amount of a silicate phosphor is added to a cation-containing solution of a barium salt or a strontium salt and is then heat-treated to obtain a surface-treated phosphor.
This method is also simple, but has a problem in that an object to be coated is limited to BaSi2O5:Pb that has relatively high water resistance. Further, there is also a problem that surface treatment is performed simply by binding the salt to the surface of the particles, and therefore a resulting coating film is not dense and is poor in water resistance or moisture resistance. Further, the heat treatment is performed at a high temperature of 700 to 1000° C., and therefore when this treatment is applied to a silicate phosphor, light-emitting characteristics are significantly deteriorated due to thermal degradation. Further, this treatment has a significant impact on silicate phosphor particles depending on the type of salt used, and various problems such as acceleration of elution arise.
In order to solve the above problems, as disclosed in Patent Literature 3, the present inventors have proposed a method for obtaining film-coated phosphor particles, in which an aluminum organic compound layer is formed as a base layer on the surface of particles of a phosphor for LEDs, a coating material layer made of a partially-hydrolyzed condensate of a silane organic metal compound (hereinafter, referred to as a “hydrolysis condensate”) is formed on the base layer, and then the phosphor particles are heat-treated at 200 to 400° C. in the atmosphere.
According to this method, it is possible to obtain phosphor particles having excellent moisture resistance and water resistance, because the aluminum organic compound previously formed as a base layer acts as a water protective film and therefore degradation caused by moisture can be prevented during the formation of the coating material layer on the base layer. However, the fact is that the other of the above problems has not yet been resolved, that is, formation of the coating film has not led to an improvement in the light-emitting characteristics of the phosphor itself.